This invention relates in general to controls for three-phase AC motors and, more particularly, to safety apparatus for controlling the direction of movement of a three-phase AC reversible direction hoist motor.
Three-phase AC motors are frequently encountered in industrial applications for driving hoists and other machinery. Such motors may, for example, be of the "squirrel-cage" induction type. These motors rotate in one direction upon the application of three-phase AC power to the motor, there conventionally being three motor leads. However, such a motor will operate in a reverse direction if any two of the three leads which supply power to the motor are reversed.
In most installations, the direction of rotation of the motor must be correctly predetermined in order for there to be proper and safe operation of machinery connected to and driven by the motor. In the case of a reversible-direction hoist, such as a chain hoist for lifting heavy loads, control of the motor is conventionally through the use of a motor control switch which may be moved to one direction for a first direction of movement of the hoist, and to another position for an opposite direction of hoist movement. As an example, the control switch may control the up and down movements of the hoist.
The direction of rotation of such motors may, therefore, be reversed by operating the control switch. However, the control switch is typically provided with indicia for the control positions, signifying the direction of movement to be effected upon movement of the control for the position thus marked. For example, a chain hoist may have a switch with positions marked "up" and "down." Whether the hoist will, in fact, move up or down if the hoist control switch is placed in these respective positions is entirely dependent, however, upon whether the AC motor wiring has been properly connected to a three-phase AC power source by a correct three lead wiring connection. If the wiring connection has been improperly carried out so as to be incorrect, the motor circuits thus connected may be said to be improperly phased. As a result, operation of the control switch to produce rotation of the motor in a desired direction will produce movement in the incorrect direction.
Now it ratherly frequently occurs that mistakes are made in wiring three-phase machinery of the present type resulting in a wiring connection of the motor circuits to an AC power distribution system which is incorrect. Such mistakes may be the result of a lack of knowledge or inexperience on the part of electricians or the result of simple inadvertence. Nevertheless, if the motor is incorrectly phased by such an improper connection, the electric motor hoist will move in the direction opposite from that selected. If the hoist control switch is moved to its "up" position, the hoisting apparatus will be lowered, presenting at the very least operator confusion and annoyance or, more likely, the possibility of damage to the hoisting machinery and the loads lifted or lowered by it. Even worse, there is also the possibility that such improper direction will endanger the safety and well-being of employees and thereby constitute a very real safety hazard. For example, rather than being raised as intended, a load may be lowered upon an employee, with consequent injury to such employee.
As an illustration of the damage to the hoisting machinery which may occur as a result of such an incorrect connection to the AC power source, it should be understood that hoists are sometimes designed so that continued operation in one direction beyond a predetermined extent may not be physically possible. Thus, in the case of a chain hoist, the operation in an incorrect direction may run the chain out of the hoist pulley with consequent damage to the hoist, notwithstanding the safety hazard implicit in such possibility.
When hoists are first placed in operation, i.e., when newly installed or after repairs, incorrect connection of such apparatus to the three-phase AC power source has resulted in the hoist not being properly phased and the resultant movement in the wrong direction in some cases has actually caused the hoists to be severely damaged or torn up. Such damage is expensive and time-consuming to correct.
Such problems of incorrect phasing of three-phase equipment have long been present and are not infrequently encountered.